This invention relates to an intrusion alarm system for premises. The invention relates more particularly to an electronic intrusion alarm system for use in both residential and industrial premises.
In modern electronic security applications, for both residential and industrial usage, there is a wide choice of alarm systems and intrusion detecting transducers. For example, there exist magnetic switch transducers which detect opening or closing of doors and windows. There are ultrasonic motion detectors, vibration sensors as well as other detecting devices. In all of these types of arrangements, whenever an alarm condition is sensed, a provision is made to provide an alarm, which may take many forms such as an activation of circuitry which automatically telphones a central office, activation of an external alarm bell, activation of special floodlights and the like. All of these known security systems require a control station from which the security system can be armed and monitored.
One such prior art security system is disclosed in the U.S. Pat. No. 3,803,576 to Dobrzanski et al. which discloses a residential alarm system which is provided with at least one normally-open contact switch which is closed by an unauthorized operation of a door or window with which it is associated. The system is designed to monitor only normally-open switch contacts and utilizes an SCR which responds to an output signal produced by the switch contacts to energize an audible alarm. Once the alarm is turned on, it remains on until the system is deactivated. The Dobrzanski et al. alarm system cannot be readily reset because of the fact that SCR's once conducting remain conducting until the anode voltage is removed. Moreover, the Dobrzanski et al. circuit suffers from the shortcoming that it is arranged to operate solely with only one type of intrusion detecting contact switch. The alarm system of Dobrzanski et al. is provided with a two time delays which allow for exit and entry without immediately activating the alarm.
An intrusion alarm system is disclosed in the U.S. Pat. No. 3,787,832 to Bolinger which includes a manually setable mechanical delay device which may be set by an authorized person ready to exit premises sought to be protected so as to allow exit without energizing the alarm system until the mechanical timer arms the system. A similar delay function is provided in the Bolinger alarm system which allows a person to enter the premises without immediately actuating the alarm, a fixed delay between sensing and actuation being provided by a thermal delay device connected in series with the alarm. The system of Bolinger provides as its sensing the sensing of a normally-closed contact switch which, when open, effects the energization of an alarm bell.
The systems disclosed in the two above-mentioned patents, while providing exit and entry delay features, have a number of disadvantages. In particular, each of these known systems is designed to operate with only one type of switch contact. That is, the respective systems are to be associated with either normally-closed contacts or normally-open contacts to the exclusion of the other type. Moreover, neither of these known systems are provided with any instrumentalities which limit the time period during which the alarm is sounded and no reset provisions have been made. Most importantly, the circuit implementations employed in these systems do not allow for the lamp self test, bell timer, and noise suppression features described herein.